Rotor craft noise cancellation system and method

ABSTRACT

Rotor noise cancellation through the use of mechanical means for a personal aerial drone vehicle. Active noise cancellation is achieved by creating an antiphase amplitude wave by modulation of the propeller blades, by utilizing embedded magnets through an electromagnetic coil encircling the propeller blades. A noise level sensor signals the rotor control system to adjust the frequency of the electromagnetic field surrounding the rotor and control the speed of the rotor. An additional method comprises of incorporating a phase lock loop within the control system configured to determine the frequencies corresponding to the rotors and generate corrective audio signals to achieve active noise cancellation.

PRIORITY CLAIMS

This application is a continuation of U.S. patent application Ser. No.16/450,926, filed on Jun. 24, 2019, the contents of which areincorporated herein by reference.

TECHNICAL FIELD

The present invention concerns rotor noise cancellation and, morespecifically, systems and methods for eliminating rotor noise foraerial, land, and water vehicles or drones.

BACKGROUND OF THE INVENTION

Active Noise Control (ANC) is a method of reducing unwanted noise byactively generating an anti-sound, cancelling out the noise by means ofan effect called phase cancellation or destructive interference. Thenoise, like any sound, is composed of alternating compression andrarefaction phases, which the human ear perceives as sound. By creatingan inverted sound, with a rarefaction phase during the noise'scompression phase and vice versa, the noise pressure wave is cancelledout, reducing the noise. Rotor based systems pose the extra challenge ofproducing rotating pressure fields that exhibit a highly complex phasestructure and are thus very difficult to cancel out using ANC.

The three main components of active noise cancellation are power, phaseand position. Without addressing all of these, it is not possible toachieve good attenuation of all noise frequencies, in all directions. Toreduce noise successfully, the noise reduction component needs to beable to generate and absorb sound within the entire targeted frequencyspan. Without correct phase, noise suppression will not work. Noisesemanating from a rotor system have a very complicated phase structure.The noise is not only generated by the rotor blades but also by theinteraction of the rotating flow with surrounding components. Toactively reduce noise, the anti-noise source needs to be inserted in theheart of the flow. When using ANC, position is vital. A noise sourcetargeted with ANC, will have noise maxima and minima unless the noiseand anti-sound can be successfully joined.

Common ANC uses microphone to assess phase and a speaker to project anantiphase in order to produce destructive oscillation waves to cancelout noise. A system may use an oscillation sensor or microphone in orderto produce an antiphase by oscillating the blades of the rotor in 180degree opposing frequency and period of the oscillation generated by therotor fan blades.

ANC is a technique most often used to control sound levels in smallspaces such as the ear cavity of noise cancelling headphones. ANCtechnology functions by processing the sound waves around the device andusing a speaker to emit identical waves but with an 180 degree phaseshift. Sound travels through the air as a pressure wave that behaveslike the sine function. The peaks of the wave are pockets ofhigh-pressure air, and the troughs are pockets of low-pressure air.Shifting the phase 180° causes the peaks of the original wave to alignwith the troughs of the modified “anti-noise” wave. When a low-pressurepocket aligns with a high-pressure pocket, they cancel each other out,reducing the noise level.

Since it is impossible to receive a sound wave signal through amicrophone, process it, and instantaneously play it back with a180-degree phase shift, it is necessary for an ANC algorithm to predictthe phase of the sound it cancels at the time that the speaker outputoccurs. A frequently used method for determining the correct output isthe least mean square (LMS) adaptive error correction algorithm. Thealgorithm calculates the speaker output by summing the product of aseries of inputs and a series of weights corresponding to each input.The weights are adjusted in each iteration of the loop based on theinput from previous iterations of the loop, a preset constant used totune the algorithm, and past output error. Over the course of severalthousand iterations, the system can learn to produce the desired outputvery accurately.

ANC is considered to be more effective because it can be adapted to anyrotor system with low power consumption. In the past, sound regulationhas not been enforced due to limited use of rotor craft, future rotorcraft traffic will only increase, vehicles will grow in both size andnumber, as a result, noise pollution control will become a requirement,especially in dense urban centers. New technical implementations thatcould allow this to occur include the progress in the development ofadvanced battery technology, and reduced prices due to the economies ofscale production, will make these rotor craft common place in dailylife.

For years, the drone market was in a nascent phase and had yet to breakinto the mainstream. Then, in 2015, drone industry growth took a majorstep forward when the Federal Aviation Administration (FAA) grantedhundreds of new exemptions for companies to operate drones in the U.S.These exemptions included several new use cases in multiple industries,such as insurance, construction, and agriculture. Each of thesescenarios demonstrate the broad range of commercial applications fordrones.

The FAA helped push drone market growth forward by formulating aregulatory framework with its consumer drone registry. Dronemanufacturers and tech suppliers are doing all they can to capitalize onthis and turn drones into a full-fledged industry. According to theresearch firm Gartner, Total drone unit sales climbed to 4.2 millionworldwide, and revenue surged 36% to $4.5 billion in 2016 alone. Inaddition, Consumer Technology Association points out that 2.4 millionpersonal drones were sold in the U.S. alone in 2016, more than doublethe 1.1 million sold in 2015.

While these indicators show that the popularity and use of drones isgradually increasing, a major tradeoff related to drone usage is thenoise pollution associated with them. Specifically, the high-pitchedsound associated with drones has started to become the central issue inmany debates. Importantly, A 2017 study by the National Aeronautics andSpace Administration (“NASA”) has found that individuals find dronenoise to be more of a nuisance than that of any ground vehicles. Infact, even where the two objects are placed at a similar volume, anentire set of 38 random individuals claimed that the drone noise wasmore of an irritant than other noises.

Currently, there is a need to reduce the noise generated by drones. Thiscan provide multiple benefits, including a stealth use for drones whileserving societal purpose of noise reduction. The invention contemplatedherein seeks to provide that solution.

The noise generated by Unmanned Aerial Vehicles (UAV) can be caused by:the propulsion system converting the energy of the fuel into the thrust;the flow of the air around the fuselage; and the vibration of thestructure as a result of the propelling force. The noise of thepropulsion system is significant during the typical flight of the UAVs.Fuel cell electrically powered UAVs operate more efficiently and createless vibration and noise compare to combustion or gas UAVs, especiallyat high altitude operations. The turbo charger of the combustion enginesis generally used during the high-altitude operations. The fuel cellsystems drawback is their low power density compare to conventionalfuels. Extra back up batteries may be used to overcome this weakness.

The propulsion system of the UAV creates structural vibrations. Inaddition, gusts of wind, fluctuation of temperature at the differentparts of the aircraft, high altitude radiation effects on compositematerials also create added vibrations. These vibrations generate noise.Often times, the approach to noise reduction is through finding ways toabsorb the structural vibrations. However, this is a double-edgedsolution, since vibrations may often be used for better energydissipation.

One of the simplest methods of limiting UAV noise is weight reduction.If the mechanisms and electronics the propellers must lift are lighter,less thrust is required, and the motors may be run more slowly.Propellers and motors are quieter when run more slowly because theycause lesser internal vibrations of the vehicle and weaker disturbancesin the air. The design and material of the propeller can also contributeto the sound produced by a UAV. Larger, slower propellers are oftenquieter than smaller, faster spinning designs. Also, more efficientpropellers need not be run as fast in order to produce sufficient liftand may therefore produce less noise. Similarly, if an external devicesuch as a shroud increases the efficiency of the propulsion system, thismay also reduce the noise level. Propellers are shaped and angled suchthat a lower-pressure region develops above the propeller and ahigher-pressure region develops below the propeller. The higher-pressureair on the bottom pushes the propeller upwards. However, instead ofgenerating lift, the high-pressure air underneath can also flow aroundthe tip of the propeller blade to reach the low-pressure region abovethe propeller, and turbulent air currents can form. Well designed, closefitting shrouds can limit these propeller tip vortices which are oftenresponsible for considerable noise and inefficiency.

SUMMARY OF THE INVENTION

In accordance with the present invention, ANC is achieved by creating anantiphase amplitude wave by modulation of the propeller blades rotation,by utilizing embedded magnets through an electromagnetic coil encirclingthe propeller blades. The embedded magnets may be affixed to thepropeller blades after the manufacture of the propeller blades or may bedesigned to be manufactured within the propeller blades. By modulatingthe rotation of the blades in response to sensing the noise produced bysaid rotation, the amplitude of the noise may be reduced.

In one embodiment, an electromagnetic coil is added to the inner rotorframe encircling the rotors, and magnets are fixed to the end of eachrotor. This causes the rotors to emit the anti-noise signal throughminute modulations of the propeller blades, which are achieved byswitching on and off a coil within the rotor frame that interacts withmagnets inside the blade tips. This method results in ANC throughmechanical means. The anti-noise comes from the exact same position asthe original noise and precisely matches its phase and rotation patternand can be applied to omnidirectional rotors.

In the preferred embodiment, two microphones are installed directlyacross from one another on the inner rim that encircles the rotors, forsensing or listening to the propeller blades as they rotate. ANC isproduced with a combined electronic field, and its associated magneticfield, and microphone element, comprising a motor driven rotor. Therotation of the rotor in the air is superimposed to a sound-pitchmodulation corresponding to a desired sound generation by the rotationof the rotor with said sound pitch. By alternating the rotor blades forpushing the air (positive compression) towards the listener and in theopposite direction respectively (negative compression) from thelistener, the same compression conditions are achieved as the propellersvibrate. By altering the sound-pitch, extremely low frequency sounds canbe generated, even below the audible range. The momentary sound pressureof the sound is thus controlled by means of an electric signal to theelectromagnet around the rotor for control of the sound-pitch of itsnegative signal—negative pressure and flow and positive signal—positivepressure and flow. This method results in ANC through use of anelectromagnetic wave applied across the rotating propellers and theirassociated magnets. The sound level of the generated sound can either becontrolled by different rotor angles or by varying the rotational speed,since both measures can influence the sound pressure and the transportedamount of air in each sound wave.

In the preferred embodiment, the system includes one or more microphonesor oscillation sensors fixed to the inner rotor frame and configured tocapture an audio signal or oscillation phase. These microphones oroscillation sensors are configured to determine one or more frequenciescorresponding to the one or more rotors and generate one or morecorrection signals corresponding to a dynamic sum of the one or morefrequencies and generates a second audio signal or oscillation bycombining the one or more correction signals or oscillations and thefirst audio signal or oscillation. Through the use of a rotor forcontrolling an oscillator which in turn controls the electrical signalsupplied through the coils which surround the rotor frame. By allowingthe rotor blades to be freely moveable, the rotor rotation can becontrolled by the sound inducing airflow back and forth throughelectrical means that can detect the angle displacement of the rotors.

The disclosed embodiments can be retrofitted to existing rotor blades,after the rotor craft has been manufactured. In lieu of microphone, anoscillation sensor can be added. This will mitigate noise interferencefrom wind and other exterior elements. With an algorithm thatcompensates for rotations per minute (RPM) and air density and speed, amicrophone will only be used as a sensing mechanism to control theelectronic oscillator to control the production of the electronic fieldthrough the coils surrounding the rotor. A phase-locked loop system canbe provided. A phase lock loop (PLL) is a control system that generatesan output signal whose phase is related to the phase of an input signal.There are several different types; the simplest is an electronic circuitconsisting of a variable frequency oscillator and a phase detector in afeedback loop. Keeping the input and output phase in lock step alsoimplies keeping the input and output frequencies the same. Consequently,in addition to synchronizing signals, a phase-locked loop can track aninput frequency, or it can generate a frequency that is a multiple ofthe input frequency.

An electromagnetic coil can reduce noise by creating a destructiveinterference by oscillating the rotor blades imbedded with magnets 180degrees out of phase. Using the Principle of Faraday's Law, the magnetsattached to the ends of each rotor blade will pass through a coil ofwire around the ducting for the rotor as they spin. An electricalcurrent will be created as the magnet passes through the coil. Tocounteract the oscillation of the rotors, an inversely proportionalcurrent will be applied in the opposite direction to the coil which willnegate the oscillation of the magnets by forcing the magnets to move 180degrees out of phase, thereby creating a destructive interference.

When a magnet travels through a coil of conductive wire, an electricalcurrent is produced, and as the rotor with imbedded magnets oscillatesin a coil it will generate an electric current as well. The current inthe coil will change direction as the poles of the magnet oscillateinside the coil. This current can be countered by adding anotherelectrical current in the opposite direction. As a result, the motion ofthe magnet will be countered by the electromagnetic field generated bythe coil, thereby canceling out noise created by the oscillation(vibration) of the rotor. The sound profile can also be changed toincrease the noise by the same means, flowing current in phase with therotors. This can also be used as a signaling device or landing cautionwarning.

Other features and aspects of the disclosed technology will becomeapparent from the following detailed description, taken in conjunctionwith the accompanying drawings, which illustrate, by way of example, thefeatures in accordance with embodiments of the disclosed technology. Thesummary is not intended to limit the scope of any inventions describedherein, which are defined solely by the claims attached hereto.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of the overall system of the present invention.

FIG. 2 is a rendering of the rotor embodiment of the present invention.

FIG. 3 is a flow diagram of the ANC embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In the preferred embodiment of the present invention, ANC will work byenergizing the perimeter coils with various wave functions to create anelectronic field, in turn to create an orthogonal magnetic field thatwill act upon the magnets associated with each of the propeller bladesor a subset of the propeller blades. The electronic wave functions mayhave varying waveforms, shapes, frequencies and may be modulated byvarious techniques well known by those of skill in the art. The rotatingpropeller blade housing may be manufactured with or supplemented by aseries of windings, suitable for creating an electronic field andassociated orthogonal magnetic field. In terms of the power of theelectronic field or its amplitude, those of skill in the art will selectvarious amplitudes sufficient in comparison to the weight and speed ofthe propellers to be modulated. Importantly, the present inventionrelies on a feedback loop. In normal operation, the fan blades aredriven by a motor and produce noise. According to the present invention,the blades include magnets and a series of electronic coils surroundingthe fan blades and associated magnets. In addition, the presentinvention includes a microphone which “listens” to the noise produced bythe rotating fan blades, and then, a control system measures said noiseand generates and applies a waveform across the electronic coils. Inother words, the control system generates a signal resulting in anelectronic field being generated along the electronic coils. In turn,the electronic coils produce a magnetic wave perpendicular to theelectronic field, wherein said magnetic wave acts upon the magnets onthe fan or propeller blades. That magnetic field causes a phase shift inthe noise propagated by the rotating blades, and in turn, the microphonepicks up that shift and then, the control system varies the electricfield until the noise propagated is minimized. Once said minimum isobtained, a phase lock loop feedback circuit may be used to maintainthat state.

Antiresonance is a pronounced minimum in the amplitude of one oscillatorat a particular frequency, accompanied by a large shift in itsoscillation phase. These frequencies are known as antiresonantfrequencies, and at these frequencies, the oscillation amplitude candrop to almost zero. Antiresonances are caused by destructiveinterference, for example between an external driving force andinteraction with another oscillator. The reduced oscillation amplitudeat an antiresonance can be regarded as due to destructive interferenceor cancellation of forces acting on the oscillator. An important resultin the theory of antiresonances is that they can be interpreted as theresonances of the system fixed at the excitation point. By utilizingdifferential equations, one of skill in the art can calculate what wavewill zero out another wave, and consequently, vary the rotational motioncharacteristics so that said waves cancel each other out so that theoverall noise emitted from the device is at an absolute minimum. Then, aphased locked loop circuit can be used to “lock in” said minimum noise.The antiresonances of a system are independent of the properties of thedriven oscillator, and they do not change if the resonance frequency ordamping coefficient of the driven oscillator are altered. This resultmakes antiresonances useful in characterizing complex coupled systemswhich cannot be easily separated into their constituent components. Theresonance frequencies of the system depend on the properties of allcomponents and their couplings and are independent of which is driven.The antiresonances, on the other hand, are dependent upon the componentbeing driven, therefore providing information about how it affects thetotal system.

FIG. 1 is a diagram of the overall system of the present invention. Inaccordance with the preferred embodiment of the present invention, therotor blades 100 are connected to an oscillator motor 102. Theoscillator motor 102 is linked to a control system 104 that controls thespeed of the oscillator motor 102. A noise controller 106 component ofthe control system 104 uses a PLL to generate and maintain an outputsignal phase in relation to the phase of an input signal, as a reactionto signals transmitted from a sensor 108. The rotor blades 100 aresurrounded by an electromagnetic coil 110 that lines the inner wall ofthe rotor frame 112. Each rotor blade 100 is embedded with a magnet 114.When the oscillator motor 102 spins the rotor blades 100, anelectromagnetic field 116 is generated between the electromagnetic coil112 and the magnets 114. The sensor 108 detects the noise level 118produced by the spinning rotor blades 100. The sensor 108 signals thenoise controller 106 to adjust the speed of the oscillator 102 throughthe control system 104. The control system 104 can adjust theelectromagnetic field 116, thereby controlling the magnetic charge 114and altering the speed of the rotating blades 100. The resultingelectromagnetic field 116 effectively generates an anti-noise wave witha variable wavelength 120. When combined with the normal noise ofwavelength 122 created by the oscillator motor 102 and rotor blades 100,the anti-noise wavelength 120 cancels out the noise (cancelled noise124), thereby producing ANC.

FIG. 2 is a rendering of the mechanical noise cancellation component ofthe present invention. In accordance with the preferred embodiment ofthe present invention, the rotor 200 wall is lined with anelectromagnetic coil 202. Located at the center of the rotor 200, is amotorized hub 204 controls the propeller blades 206. The tip of eachrotor blade 206 has an embedded magnet 208. These magnets create anelectromagnetic charge through the coil 202, generating anelectromagnetic field and creating an antiphase amplitude wave tomodulate the propeller blades 206 and achieve ANC. Importantly, controlsystem 104 may “learn” what wave functions produce ANC, so that forvarious configurations of fan blades, weights and environmentalconditions, ANC may be achieved at all times eventually. Because thepresent invention uses a sensor or microphone or series of each 108,real life conditions may be processed, and a minimum noise profiledeveloped for each fan blade—rotor combination. And once an optimalnoise condition is obtained, a PLL may be utilized within the controlsystem 104 and noise controller 106 to maintain that state.

FIG. 3 is a diagram of the overall feedback system according to thepresent invention, which constitutes the controller of the propulsionsensor controller oscillator of the present invention. In accordancewith the preferred embodiment of the present invention, a propulsiondevice 300 is connected to an oscillation sensor 302. The oscillationsensor 302 communicates with a noise cancellation controller 304. Thenoise cancellation controller 304 generates an anti-noise wavelengthwithin the electromagnetic coil oscillator 306. The anti-noisewavelength generated from the electromagnetic coil oscillator 306controls the propulsion device 300. Accordingly, the magnets spinningthemselves generate a magnetic field which translates to an electronicfield which the oscillation sensor 302 may react to; or, the noisecancellation controller 304 may generate an electronic field to createan anti-noise wave, electromagnetic coil oscillator signal 306 to actupon the propulsion device 300. While the arrows shown depict the orderof operation, it is true that the process may run in both directions sothat the magnets 114 are both generators of energy and may be acted uponas they rotate by the coils 202 contained within coil unit 110. In bothinstances, the goal is a minimal output of noise which a PLL can be usedto lock in. In this manner, it is equally true that the magnets 114induce electrical currents along the coils 202 that may be used tocharge batteries associated with powering motorized hub 204 andconversely, the coils 202 may be energized to create a magnetic fieldfor retarding the movement of the magnets 114 to alter the fan bladerotation for achieving ANC.

While various embodiments of the disclosed technology have beendescribed above, it should be understood that they have been presentedby way of example only, and not of limitation. Likewise, the variousdiagrams may depict an example architectural or other configuration forthe disclosed technology, which is done to aid in understanding thefeatures and functionality that may be included in the disclosedtechnology. The disclosed technology is not restricted to theillustrated example architectures or configurations, but the desiredfeatures may be implemented using a variety of alternative architecturesand configurations. Indeed, it will be apparent to one of skill in theart how alternative functional, logical or physical partitioning andconfigurations may be implemented to implement the desired features ofthe technology disclosed herein. Also, a multitude of differentconstituent module names other than those depicted herein may be appliedto the various partitions. Additionally, with regard to flow diagrams,operational descriptions and method claims, the order in which the stepsare presented herein shall not mandate that various embodiments beimplemented to perform the recited functionality in the same orderunless the context dictates otherwise.

Although the disclosed technology is described above in terms of variousexemplary embodiments and implementations, it should be understood thatthe various features, aspects and functionality described in one or moreof the individual embodiments are not limited in their applicability tothe particular embodiment with which they are described, but instead maybe applied, alone or in various combinations, to one or more of theother embodiments of the disclosed technology, whether or not suchembodiments are described and whether or not such features are presentedas being a part of a described embodiment. Thus, the breadth and scopeof the technology disclosed herein should not be limited by any of theabove-described exemplary embodiments.

Terms and phrases used in this document, and variations thereof, unlessotherwise expressly stated, should be construed as open ended as opposedto limiting. As examples of the foregoing: the term “including” shouldbe read as meaning “including, without limitation” or the like; the term“example” is used to provide exemplary instances of the item indiscussion, not an exhaustive or limiting list thereof; the terms “a” or“an” should be read as meaning “at least one,” “one or more” or thelike; and adjectives such as “conventional,” “traditional,” “normal,”“standard,” “known” and terms of similar meaning should not be construedas limiting the item described to a given time period or to an itemavailable as of a given time, but instead should be read to encompassconventional, traditional, normal, or standard technologies that may beavailable or known now or at any time in the future. Likewise, wherethis document refers to technologies that would be apparent or known toone of ordinary skill in the art, such technologies encompass thoseapparent or known to the skilled artisan now or at any time in thefuture.

The presence of broadening words and phrases such as “one or more,” “atleast,” “but not limited to” or other like phrases in some instancesshall not be read to mean that the narrower case is intended or requiredin instances where such broadening phrases may be absent. The use of theterm “module” does not imply that the components or functionalitydescribed or claimed as part of the module are all configured in acommon package. Indeed, any or all of the various components of amodule, whether control logic or other components, may be combined in asingle package or separately maintained and can further be distributedin multiple groupings or packages or across multiple locations.

Additionally, the various embodiments set forth herein are described interms of exemplary block diagrams, flow charts and other illustrations.As will become apparent to one of ordinary skill in the art afterreading this document, the illustrated embodiments and their variousalternatives may be implemented without confinement to the illustratedexamples. For example, block diagrams and their accompanying descriptionshould not be construed as mandating a particular architecture orconfiguration.

1. A rotating fan system with propellers wherein said propellers includemagnetic portions and wherein said magnetic portions interact with aproximal electromagnet for noise reduction wherein said propellers arerotated by a motor independent of energy generated by said proximalelectromagnet and wherein said proximal electromagnet createsinterference acting upon said rotation of said propellers.
 2. The systemof claim 1 wherein said propellers are manufactured with magneticportions.
 3. The system of claim 1 wherein magnets are attached to saidpropellers.
 4. The system of claim 1 wherein electronic waves are inputto said electromagnet to modify the movement of said propellers.
 5. Amethod for reducing propeller noise by magnetizing propellers whereinsaid propellers are rotated by a motor independent of energy generatedby a proximal electromagnet and wherein said proximal electromagnetcreates interference acting upon said rotation of said propellers andapplying a magnetic field to said propellers to cancel noise.
 6. Themethod of claim 5 wherein said propellers are manufactured with magneticportions.
 7. The method of claim 5 wherein magnets are attached to saidpropellers.
 8. The method of claim 5 wherein electronic waves are inputto said electromagnet to modify the movement of said propellers.